1. Field of the Invention
The present invention relates to a magnetic recording medium on which information is magnetically recorded by a magnetic recording-reproducing unit. More particularly, it relates to a magnetic recording medium such as magnetic recording disk, magnetic tape, magnetic drum, and magnetic card capable of storing information in the form of magnetic spots, the information being coded by the magnetic disk unit, magnetic tape unit, etc. which are used as the external memory unit of computers.
2. Description of the Prior Art
With the recent advance in computer technologies, a variety of magnetic recording-reproducing units have come into general use as the external storage of computers. The magnetic recording-reproducing units include magnetic disk recording-reproducing units, magnetic tape recording-reproducing units, magnetic drum recording-reproducing units, and magnetic card recording-reproducing units, which are designed to magnetically store the information which has been processed by the arithmetic logical unit of the computer. These units employ magnetic disk, magnetic drum, magnetic tape, and magnetic card as the information recording medium.
The magnetic recording medium as mentioned above consists of a non-magnetic substrate and a magnetic layer formed on the surface thereof. The magnetic layer is covered with a protective layer having good lubricity. The quality of the magnetic layer affects the amount of information that can be stored, and the quality of the protective layer affects the lubricity between the magnetic recording medium and the magnetic head installed for recording and reproducing in the magnetic recording-reproducing unit. The protective layer, therefore, plays an important role in improving the durability and life of both the magnetic recording medium and the magnetic head. For this reason, many attempts have been made so far to improve the magnetic layer and the protective layer.
As a result of such attempts, there was proposed a new magnetic recording medium by Fuji Photo Film Co., Ltd. (see U.S. Pat. No. 3,717,504). According to the disclosure, this magnetic recording medium comprises "a non-magnetizable substrate, a magnetic recording layer having a thickness greater than 0.05 micron plated onto one surface of said substrate, a first protective layer consisting essentially of a nickel film plated on the exposed surface of said magnetic recording layer and having a thickness less than 0.2 micron and a second protective layer consisting essentially of a rhodium film plated on the exposed surface of said first protective layer and having a thickness greater than 0.02 micron."
There was also proposed "a magnetic recording medium" by Hitachi Ltd., as disclosed in Japanese Patent Laid-open No. 47926/1981 (Apr. 30, 1981). According to the disclosure, it is composed of a substrate 1, a backing layer 2, a magnetic layer 3, a first protective layer 4, and a second protective layer 5, as shown in FIG. 1. The substrate 1 is made of an aluminum (Al) alloy or magnesium (Mg) alloy. The backing layer 2, which has a mirror-finish surface, is formed by plating the substrate 1 with a copper (Cu)-tin (Sn) alloy, nickel (Ni)-tin (Sn) alloy, nickel (Ni)-phosphorus (P) alloy, or copper (Cu)-zinc (Zn) alloy. The magnetic layer 3 is formed by plating the backing layer 2 with a magnetic cobalt (Co)-nickel (Ni)-phosphorus (P) alloy or the like. The first protective layer 4, which is intended to protect the magnetic layer from corrosion, is formed by plating the magnetic layer 3 with a nickel (Ni)-phosphorus (P) alloy. The second protective layer 5 is formed by applying a fluorinated hydrocarbon such as perfluoroalkyl polyether to the first protective layer 4.
The magnetic recording medium having the laminate structure as mentioned above functions in the following manner. As the magnetic recording medium slidingly passes across the magnetic head, data is recorded in the magnetic layer 3 or data stored in the magnetic layer 3 is retrieved. During the passage of the magnetic recording medium, the second protective layer 5, which is a fluorinated hydrocarbon, provides lubricity to protect the magnetic layer 3 from wear by the magnetic head.
The above-mentioned conventional magnetic recording medium has to have the first and second protective layers on the surface of the magnetic layer. Because of the dual protective layers, the conventional magnetic recording medium is inevitably thicker than necessary.
In the second example of the prior art, the outermost second protective layer of fluorinated hydrocarbon such as perfluoroalkyl polyether is not satisfactory for protection for a long period of time. This leads to poor reliability. In addition, the dual protective layers need additional production steps which lead to an increase in production cost.